Optical fiber technology is well suited for communications applications because optical fibers have a wide transmission bandwidth and relatively low attenuation. In most applications, the electrical signal is first converted into an optical signal via a light emitting device such as a semiconductor laser. The optical signal is in turn coupled into an optical fiber for transmission of the optical signal. Conventionally, optical coupling assemblies have been designed in such a way that the light emitting device, all optical components, and the fiber are all aligned in line along the path of light. An advantage of this alignment scheme is that the optical assemblies are relative simple and can be made with only one simple optical component such as a ball lens. However, when the fiber is not placed in line with the light emitting device, the optical coupling assemblies must be well designed to achieve efficient optical coupling.
Recent development and progress in high speed vertical cavity surface emitting laser (“VCSEL”) technology and the rise of metropolitan fiber communication market have combined to create a need for high-speed optical transmitting devices based on VCSEL technology and relevant optical coupling assemblies. However, the use of VCSEL technology has imposed strict requirements for high power coupling efficiency and high mode coupling efficiency within optical coupling assemblies. This is because VCSEL devices convert the received electrical signal into different laser transmission modes, each having a different spatial distribution. The transmitted optical signal can be properly received only when all laser modes are coupled with even loss. Therefore, proper design of the optical coupling assemblies for efficient optical coupling from light emitting/receiving devices to fiber has been a persistent challenge.
Coupling assemblies should also allow large optical alignment tolerances to be induced by the manufacturing process. Alignment problems during assembly are exacerbated by the trends toward miniaturization of over-all size of optical transmitting/receiving devices and toward lower cost materials. The miniaturization of optical assemblies themselves requires that the optical coupling must be completed with high efficiency within a small volume. The volume or dimension requirement, however, limits the size and number of optical components that can be used in an assembly.
Notwithstanding the trend-toward miniaturization, a complicated optical assembly is generally necessary in order to achieve high power coupling and high mode coupling between two orthogonal directions, such as vertical and horizontal direction. Such optical coupling assemblies are usually bulky and require multiple optical components. This imposes additional assembling and alignment difficulties and cost on the manufacturing and is at odds with the goal of miniaturization.
In addition, optical devices, including vertical or edge emitting devices, may need to be monitored to ensure that the transmitted beam has certain characteristics. This requires a monitoring unit to be positioned within the light path of the optical devices, thus adding complexity in the optical coupling assemblies and additional space requirement to accommodate the monitoring unit itself.
Accordingly, there is a need for a compact optical assembly that incorporates VCSEL technology and techniques for efficient coupling between the laser and the fiber. There is a further need for an optical assembly that is easy to manufacture, assemble and align. There is a further need for an optical coupling assembly that has a small footprint to facilitate its use in small volume optical assemblies.